Optically clear biaxially oriented polyester film with anti-iridescent primer layer

ABSTRACT

A highly optically clear, composite film has a predominantly thermoplastic polyester base layer of A/B/C-layered structure and a polyester and polyurethane blend primer layer Preferably core layer B of the base layer is substantially particle-free polyester and outer layers A and C include organic and/or inorganic particles. The primer layer is crosslinked using a carbodiimide crosslinking agent. The primer layer and base layer composite can be laminated with a protective layer of primarily acrylic polymer for a solar control film. Functional additives, such as UV light blockers can be present in layers of the composite and solar control films. The primer layer combination of polyurethane and crosslinking by carbodiimide, provides the acrylic coated polyester solar control film with notably reduced iridescence and durable adhesion between polyester and acrylic layers in moist and warm service conditions.

This application claims the benefit of U.S. Provisional Application No.61/514,280 filed Aug. 2, 2011.

FIELD OF THE INVENTION

The present disclosure is generally related to optically clear biaxiallyoriented polyester film with at least one primer layer, having excellentadhesive properties to acrylic coating material over the primer layereven under harsh conditions, such as high moisture environment. Thepolyester film also reduces iridescent appearance after the polyesterfilm is coated with acrylic material such as hard coating. The polyesterfilm may have UV blocking and weatherable properties and may bepreferably used for window film, display film, outside clear label,outside signage and photo voltaic application.

BACKGROUND OF THE INVENTION

A commercially and technically important utility for biaxially orientedpolyester films is serving as component of many articles such as foodpackaging, printing media, electrical insulation, optical and the otherindustrial uses. The thermal stability, dimensional stability, chemicalresistance, relative high surface energy, optical clarity as well ascost effectiveness of biaxially oriented polyester films are beneficialfor typical end use applications. Regarding optical clarity, biaxiallyoriented polyster films can be used for instance as a substrate ofoptical products such as window films, display parts, touch screen,eyewear, including visors, goggles, and spectacles, lenses, sunscreens,labels and photovoltaic materials. Typically applications will involveplacing optically clear acrylic coating material onto the biaxiallyoriented polyester films.

Although, biaxially oriented polyester film, and acrylic coating eachare optically clear, the composite film of biaxially oriented polyesterfilm coated with acrylic material can have adverse optical properties.For example, the film can exhibit excessive iridescence. The refractiveindex (“RI”) of an acrylic coating material can be different from the RIof a biaxially oriented polyester film, e.g. biaxially polyethyleneterephthalate (PET) film that is about 1.66. The difference betweenthese refractive indices causes optical interference of light raysreflected from the surface between the acrylic layer and the polyesterlayer. This interference produces a rippled iridescent appearancethrough the spectral reflectance of the acrylic material-coatedpolyester film. Iridescence on the acrylic material coated polyesterfilm is very evident under spectral light of fluorescent lamps becausesuch light has a sharp distribution of luminescence that interferes withthe rippled spectral reflectance of the acrylic material coatedpolyester film.

Iridescence is reduced or does not occur if the film is hazy becauselight is scattered. Hazy film is not desirable for many optically clearend use applications. Furthermore, the use of fluorescent lighting inplace of incandescent lighting is increasing due energy conservationefforts. Consequently, the iridescence of the acrylic material-coatedpolyester film can distort or block the view through the film or detractfrom the desired aesthetic appearance of the article comprising thefilm. Accordingly, the ability to reduce iridescence is gainingimportance.

Another problem with the acrylic material-coated polyester films isadhesion between layers of acrylic material and the polyester layer. Ingeneral, biaxially oriented polyester film has highly crystalizedsurface that makes the polyester difficult to adhere to such acrylicmaterial coating layer. To overcome this drawback, a primer layer issometimes used between polyester and acrylic polymer layers to improveadhesion.

Japanese Patent Publication Number JP 2004-299101 of YOKOTA SUNAO etal., entitled “Transparent Laminated Film for Surface Protection” isdirected to a transparent composite film with a 10-250 μm thick baselayer of biaxially stretched polyester and a 3-20 μm thick hard coatlayer of acrylic polymer providing at least 90% light transmission. Thefilm is for laminating to a surface of an article, such as a flat paneldisplay member, a nameplate, a window and the like, to protect thearticle from scratching or other damage.

US patent application 2008/0038539 of Yokota et al. discloses acomposite film having a core layer sandwiched between outer layers toform a base polyester layer. A coating layer containing anti-iridescentmaterial covers one side of the base polyester layer and an acryliccoating is disposed over the anti-iridescent coating layer. US '539discloses the effect achieving anti-iridescence by optimizing RI and thecoating layer thickness to minimize infringement of reflection lightwhich causes ripples of the spectral reflectance.

However, these technical references do not fulfill the needs of modernindustries for less or no iridescence and more robust adhesion undervery harsh condition such as moisture exposure. The disclosed filmsabove have been found to provide a hard coat with only moderate adhesionto the base layer, especially after exposure to heat and moisture.

SUMMARY OF THE INVENTION

This invention relates to a highly optically clear, composite filmhaving a predominantly thermoplastic polyester base layer and a primerlayer of a polyester and polyurethane blend composition. The base layerpreferably has an A/B/C layered structure with a substantiallyparticle-free core layer B of polyester and outer layers A and C ofpolyester containing nonpolyester, organic and/or inorganic particles.The primer layer is preferably applied to the base layer from solutionthat is organic solvent-free and is crosslinked using a carbodiimidecrosslinking agent. The primer layer and base layer composite can belaminated with a protective layer of primarily acrylic polymer, forexample to obtain a solar control film. Various layers of the compositeand solar control films can contain effective amounts of functionaladditives, such as UV light blockers. Polyurethane in the primer,especially in combination with crosslinking by the carbodiimide,provides the acrylic coated polyester base solar control film withnotably reduced iridescence and durable adhesion between polyester andacrylic layers in moist and warm service conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation cross section view of a composite film accordingto an embodiment of this invention.

FIG. 2 is an elevation cross section view of a solar control filmincluding the composite film of FIG. 1 and a protective layer accordingto another embodiment of this invention.

DETAILED DESCRIPTION OF THE INVENTION

A basic embodiment of this invention is understood with reference toFIG. 1 showing a cross section of novel composite film 10. Thiscomposite film includes a base layer 5 and a primer layer 2. The baselayer is preferably formed of three sublayers, namely core layer 6, andouter layers 4 and 8. Each of the core and outer layers is predominantlypolyester and can include other components, such as “UV” (ultravioletlight) blocking additives and particles. In another embodiment shown inFIG. 2, a solar film 20 includes a protective layer 9 primarily ofpreferably an acrylic polymer. The protective layer 9 is positioned onthe composite film in contact with the primer layer 2.

In the present invention, the primer layer contains polymeric binder asa major component of the primer layer. The polymeric binder may beselected from, but not limited to, polyester, acrylic, polyurethane orthe mixture thereof. It is preferred to select polyester binder becauseits RI is similar to that of the polyester base layer and thus ishelpful to reduce iridescence in the manner as described in US2008/0038539. Also the polyester binder is very compatible with the basepolyester layer to provide preferred adhesion, in general. A preferredraw material for the polyester binder is a 30% solids dispersion, in 2%propanol, aqueous solution commercially available under the name Eastek®1200 (Eastman Chemicals Company, Kingsport, Tenn.). This polyester hasinherent viscosity of 0.34-0.42, glass transition temperature (Tg) of63° C. and softening point 65° C.

It now has been discovered that very small polyurethane particles,particularly inversely synthesized aliphatic type polyurethane, in theprimer layer 2 provide excellent iridescence canceling. Without wishingto be bound by a particular theory it is thought that the polyurethaneparticle anti-iridescent component scatters light impinging on the film.The iridescence canceling performance is highly effective. Hence,without significantly increasing haze, iridescence normally observedwith an acrylic polymer coated polyester film is reduced orsubstantially completely prevented. Consequently use of polyurethaneparticles in the primer layer controls iridescence and maintainsexcellent optical clarity and thus is different from conventionalanti-iridescent techniques described in the prior art such as US2008/0038539.

A traditional approach to reducing iridescence in composite filmutilizes refractive index matching techniques. The refractive indices ofthe acrylic material layer and the base biaxially oriented polyesterfilm layer are distinctly different. The refractive index differencegenerates iridescence, as mentioned above. Selecting a primercomposition having a RI that matches and complements those of the twolayers could reduce iridescence by canceling the effect of disparaterefractive indices.

It is contemplated that low iridescence from the polyurethane accordingto this invention is caused by a light scattering phenomenon (withoutincreasing haze) rather than or in combination with refractive indexmatching. This is remarkable because the polyurethane can be utilized ata particle size and concentration low enough to allow excellent opticalclarity of the overall film yet provide good anti-iridescenceperformance. The implications of this theory include that a polyurethanefine particle dispersion in a primer layer can be utilized to reduceiridescence in a wide variety combinations of base layer and the overcoating layer materials. Moreover, low iridescence can be obtainedwithout constraining the primer layer to have a particular refractiveindex that complements the refractive indices of the other compositefilm layers, although the combination of the refractive index matchingmay be preferred.

The preferred iridescence-reducing component polyurethane particle is aninversely synthesized aliphatic polyurethane. The term “inverselysynthesized aliphatic polyurethane” means that the polyurethane isformed by a process in which (i) non-aromatic organic polyisocyanate andnon-aromatic organic polyol are reacted to form a polyurethane polymer,(ii) a neutralizing agent, such as a tertiary amine, is added to waterto form an aqueous solution, and then (iii) the polyurethane polymer isadded to and dispersed in the aqueous neutralizing agent solution. Apreferred inversely produced polyurethane material is commerciallyavailable under the name Neorez R1010 (DSM NeoResins B.V., Waalwijk,Netherlands).

It should be understood that use of inversely synthesized aliphaticpolyurethane particles interrelates successfully with several importantperformance parameters of optically clear and solar control films. Inaddition to exhibiting little or no iridescence, these films are calledupon to have high cohesive strength for durability and to be highlytransparent. All of these properties can be achieved by thesepolyurethane particles employed in a primer layer together withpolymeric binder and a carbodiimide crosslinking agent in properproportions. The polymeric binder provides structural integrity andserves as matrix for the dispersed phase of polyurethane particles. Thecrosslinking agent transforms the binder to a rigid network. If toolittle binder or crosslinking agent is present, the primer will be tooweak to durably hold the protective layer to the base layer.Delamination can occur. The polyurethane particles cancel undesirableiridescence. If too little inversely synthesized aliphatic polyurethaneparticles are present, iridescence can occur. Conversely, if there istoo much of binder, polyurethane or crosslinking agent, or the primerlayer is toothick, the film can become excessively hazy and thusunsuitable for optical or solar control film utilities. The descriptionand examples of this disclosure provide guidance for selecting relativeamounts of the primer components for successful practice of thisinvention. The artisan of ordinary skill will be able to adjustproportions of primer components and primer layer thickness according tothe principles set forth herein to effectively apply the inventionwithout undue experimentation.

The desired particle size of the polyurethane particles within theprimer layer is about 1 nm to about 100 nm, preferably about 1 nm toabout 60 nm. If the particle size is larger than 100 nm, the coated filmsurface can develop a grainy appearance. Additionally, the haze valuecan exceed the less than 3% desired limit for the coated polyester film(i.e., the base layer/primer/acrylic polymer layer composite), therebyreducing the optical clarity. If the particle size is smaller than 1 nm,the anti-iridescing properties may not be achieved. The content of thepolyurethane particles in the primer layer is about 0.1 to 2.5 wt %,preferably about 0.1 to about 1.5 wt %. If the content is less than 0.1wt % there may not be enough particles to effectively reduce oreliminate iridescence. If the content is more than 2.5 wt %, the coatedfilm surface may exceed a less than 3% haze value, thereby reducingoptical clarity. The urethane particles are preferably uniformlydispersed within the primer layer. The primer layer should be of aconsistent dry coating weight and thickness with adequate unagglomerateddispersed particles to sufficiently provide an optically clear coatingwith a haze value. Primer layer-coated base layer (i.e., without anacrylic polymer layer) of this invention has a haze value of less than4%. The preferred thickness of the primer layer is about 0.03 to 0.15μm, more preferably, 0.07 to 0.12 μm. If the thickness is less than 0.03μm, the desired adhesiveness and anti-iridescence effect may not beachieved. If the thickness is more than 0.15 μm, then presence of theprimer can detract from overall optical clarity and the desiredanti-iridescence effect may not be achieved.

As another embodiment of this invention, great preference is given tostrengthening the primer layer and the bond between polyester film layerand the acrylic polymer layer by crosslinking the primer layer.Crosslinking strengthens the primer layer by forming the polymericbinder component of the primer layer into a permanent, rigid, networkstructure. It also produces chemical links between the polymeric binderand the polyester base layer. Crosslinking can be accomplished duringand/or after drying solvent from the wet primer layer. Typically acrosslinking agent is added to the primer composition to catalyze thereaction. Many conventional crosslinking agents for reactingpolyurethane and polyester may be used, such as carbodiimide, melamine,aziridine, glyoxal, oxazoline and mixtures thereof.

It has been found that a carbodiimide crosslinking agent providesexcellent adhesive strength while very effectively preserving theanti-iridescence property afforded by the polyurethane/polyester binderblend. Carbodiimides have the chemical formula R₁N═C═NR₂ in which R₁ andR₂ are hydrogen or hydrocarbon radicals. Additional benefits of usingcarbdimide crosslinker include improved adhesive bonding of film layers,useful potlife, low toxicity, improved chemical resistance andcrosslinking at ambient conditions. A preferred carbodiimidecrosslinking agent is Solucote® XL1 (DSM NeoResins B.V., Waalwijk,Netherlands). Preferably the crosslinking agent is added to the primercoating liquid and mixed to uniform concentration. The preferred contentof the crosslinking agent in the primer layer is about 1 wt % to 5 wt %,more preferably, about 2 to 3 wt %. If the content is less than 1 wt %,the organic solvent resistance properties, the inorganic solventresistance properties and the required adhesive strength of the primerlayer will not be achieved. If the content is more than 5 wt %,excessive crosslinking can produce haze value of the base layer/primerlayer film greater than 4%, thereby reducing optical clarity.

Inversely synthesized aliphatic polyurethane dispersion and thecrosslinking agent are mixed with the polymeric binder dispersion inappropriate preselected ratios to form a primer coating liquid. Thecoating liquid can be applied to a surface of the base layer byconventional coating methods such as dip coating, doctoring, spraying,rod coating and the like. Rod coating is preferred. Followingapplication, the primer coating liquid is dried by heating and lowhumidity ventilation to remove liquid dispersing medium (mainly water),and leave a solid content of the primer layer on the base polyesterlayer. Such coating processes can be done after the biaxially orientedpolyester film is made, or continuously (i.e., in line) with thebiaxially oriented polyester film fabrication. The in line method ispreferred to reduce the number of steps and cost of coating.

Examples of polyester suitable to form the base of the biaxiallyoriented polyester film are, polyethylene terephthalate (PET),polyethylene naphthalate (PEN), polybutylene terephthalate (PBT),polyethylene isophthalate (IPET) and blends or co-polymers thereof. Apreferred polyester is PET because of its good balance of cost andperformance.

The base layer of biaxially oriented polyester film may have a monolayer structure or a multi layer structure such as A/B, A/B/A or A/B/C.A/B/A or A/B/C structures are preferred in which wherein the core layerB is a polymer layer substantially free of particles and layers A and Ceach independently can contain organic and/or inorganic particles. Corelayer B should contain no particles to achieve the preferred opticalclarity of at most about 3% haze value and more preferably at most 2%.The outer layers A and C may have desired slip agents such as organicand/or inorganic particles, as disclosed in US patent application2008/0038539 hereby incorporated herein.

The biaxially oriented polyester film generally has a thickness of from1 to 500 μm, preferably from 5 to 350 μm. A film thickness of 10 to 50μm may be preferred for some applications, such as for solar window filmuse.

The biaxially oriented polyester film can be produced by anyconventional method, such as sequential stretching or simultaneousstretching. In an example of the fabrication process, raw materialpolyester pellets and additives are fed to a melt processor, such as amixing extruder. The molten material, including the additives, isextruded through a slot die and quenched on a chill roll, in the form ofa substantively amorphous film. The film may then be reheated andstretched longitudinally and transversely or transversely andlongitudinally, or longitudinally, transversely, and againlongitudinally and/or transversely. Temperatures during stretching aregenerally above the Tg of the film polymer by about 10 to 60° C.Preferably, the longitudinal stretching ratio is from 2 to 6, morepreferably from 3 to 4.5. Preferably, the transverse stretching ratio isfrom 2 to 5, more preferably from 3 to 4.5. Preferably, any secondlongitudinal or transverse stretching is carried out at a ratio of from1.1 to 5. The first longitudinal stretching may also be carried out atthe same time as the transverse stretching (simultaneous stretching).Heat setting of the film may follow at an oven temperature of about 180to 260° C., preferably about 220 to 250° C. The film may then be cooledand wound up.

The biaxially oriented polyester film may contain other additives suchas, but not limited to, UV stabilizer, hydrolysis resistant agent,optical brightener, frame retardant agent, anti-oxidation agent.Especially for outdoor applications such as solar window film or photovoltaic application, it is preferable that the polyester film containsUV stabilizer to protect the film itself and/or protect article behindthe film from UV light. Various terms such as “UV light blocker” “UVblocking additive”, “UV stabilizer”, “UV absorber”, “UV agent” and thelike used herein are to be construed interchangeably as referring tocomponents included the base layer to control the effect of ultravioletlight incident thereon.

Preferred UV absorbers include 2-hydroxybenzotriazoles, benzoxazinonesand the triazines. A more preferred UV absorber is2,4-bis-biphenyl-642-hydroxy-4-(2-ethyl-hexyloxy)phenyl]-1,3,5-triazinein terms of the weatherability and UV resistance performance. Thecontent of UV absorber may be 0.1 to 2 wt. %. Less than 0.1 wt. % is notenough to be effective, more than 2 wt. % may increase haze, yellowcolor, affect the mechanical properties of the film, and may createprocessing issues such as generating undesirable gaseous byproducts andcausing migration to the surface (i.e., “blooming”) of the UV absorber.Preferably, the polyester film includes 0.1 to 2 wt. % UV absorber; morepreferably in the range of 0.5 to 1.5%.

The biaxially oriented polyester film with primer of this invention canbe used as a substrate on which is deposited a coating of acrylicpolymer as mentioned above and seen in FIG. 2. The acrylic polymer layeris preferred to be optically clear and primarily to provide a hard,strong, impact resistant barrier against physical and chemical attackfrom environmental conditions to which the desired application of thisinvention can be exposed in service. That is, such protective layerreduces damage to the film from scratching, denting, moisture,atmospheric-borne contaminants, dirt and permits wash-and-wipe cleaningof the exposed film surface. The preferred thickness of the acrylicpolymer layer is 2 to 5 μm. Coating layer thickness exceeding 5 μm canadversely affect refractive indexand cause iridescence. If the thicknessis smaller than 2 μm, the chemical resistance properties of the acrylicpolymer layer can be diminished.

Acrylic polymers for the protective layer of this invention have repeatunits that are derivatives of acrylic acid or substituted acrylic acid.That is, the acrylic polymer is a polymer comprising polymerized unitsof the following formula (I)

in which X═H, for an acrylic acid derivative, or an alkyl group for analkyl acrylic acid derivative, such as CH₃ for a methacrylic acidderivative. Typically R is an alkyl group, a glycidyl group or ahydroxyalkyl group. Representative acrylic polymers include polymethylmethacrylate, polyethyl methacrylate, polybutyl methacrylate,polyglycidyl methacrylate, polyhydroxyethyl methacrylate, polymethylacrylate, polyethyl acrylate, polybutyl acrylate, polyglycidyl acrylate,polyhydroxyethyl acrylate and mixtures of these. A protective layer ofpredominantly acrylic polymer typically has a refractive index of atmost about 1.54, and frequently about 1.48 to about 1.54.

The acrylic polymer layer can include functional additives for specificpurposes. Typical additives include antioxidants, impact resistancemodifiers, surfactants, light blocking additives and the like.Preferably the acrylic layer for the desired application such as solarcontrol films contain light blocking additives, particularly UV lightblocking materials. The additives are usually present in minorquantities to avoid degrading optical clarity of the acrylic polymerlayer. Typically the acrylic polymer layer contains a total of less thanabout 1 wt % of additives.

Frequently, coated polyester films are placed on products to provide asolar control function by blocking, absorbing and/or otherwise resistingthe transmission of selective wavelengths of light. These phenomena aresometimes hereinafter collectively referred to as “light blocking”.Ultraviolet light blocking is a significant utility for coated polyesterbase solar control films. PET, which is a most desired polyester, alonedoes not resist transmission of ultraviolet (“UV”) light very well. UVlight blocking is typically improved by placing a coat of an effectiveUV light blocking material on a PET core layer of a composite film. Anexample of such a material is a polymeric coating, for example, a hardcoat such as an acrylic polymer that contains a uniformly dispersed UVlight blocking composition. The hard coat also physically protects thePET base layer with enhanced impact resistance, abrasion resistance andlike properties that lower the risk of damage from denting, scratchingand similar other environmental assaults.

In one embodiment of the invention, the acrylic polymer layer includesUV light blocking materials. In that case, the acrylic polymer layergenerally comprises greater than about 60 wt. %, preferably greater thanabout 80 wt. %, more preferably, greater than about 95 wt. %, and mostpreferably greater than about 98 wt. % of an acrylic polymer and 0 toabout 2 wt % preferably about 0.05-1 wt. %, more preferably about0.1-0.5 wt. %, and most preferably about 0.2-0.4 wt. % of ultravioletlight blocking component.

The acrylic polymer can be applied to the primer layer from a solutionof the acrylic polymer dissolved in organic solvent. Although theorganic solvent is substantially completely removed from the acrylicpolymer after lamination, trace amounts of solvent can remain in theacrylic polymer layer. Not uncommonly, the compositions in the primerlayer are soluble in organic media. Over time, the very slight butfinite residual solvent in adjacent acrylic polymer layer can weaken theprimer layer. After exposure to heat and moisture under environmentalservice conditions, cracking and flaking off of the weakened acrylicpolymer layer can occur. A system for adhering acrylic polymer layerapplied from organic solvent media onto optically clear solar controlfilms is much needed in the industry. Therefore, superior organicsolvent resistance of the primer layer is also desired and is achievedby crosslinking agent mentioned above.

The acrylic polymer layer can be laminated to the biaxially orientedpolyester film by various known methods. The term “laminate” is usedherein to mean the generic permanent joining of layers to form acomposite structure and is not limited to any specific method. Forexample, the acrylic polymer layer can be preformed as a sheet andlaminated onto the primer layer using heat and pressure. Acrylic polymercan also be extruded onto the primer layer. Preferably, the acrylicpolymer can be deposited onto the primer layer from a solution. Examplesof solution application methods include doctoring, spraying, painting,dipping, and rod coating techniques. Following application of solution,the solvent is removed by conventional techniques such as heat and/orvacuum treatments. A preferred material for making the acrylic polymerlayer of this invention is an acrylic polymer with a UV cure intiatorsolution in methyl ethyl ketone and isopropyl alcohol solvent. The cureinitiator is one which can be activated by exposure to UV radiation.

Ideally, all of the solvent is removed to leave a dry, e.g. hard coat ofthe acrylic polymer for the solar control film or display film such astouch screen.After the solvent removal step, it is not unusual for traceamount of the solvent to remain in the seemingly dry acrylic polymerlayer. The primer according to this invention can withstand thesolubilizing effect of the presence of such trace residual solvent. Itis thus able to create a durable and strong bond between the acrylicpolymer layer and the polyester base layer.

Consequently, the acrylic polymer layer will resist cracking, chipping,flaking and peeling from the base layer for extended duration.

Among the contemplated embodiments of this invention are included thefollowing.

1. Optically clear biaxially oriented polyester film comprising ananti-iridescent primer layer comprising, (A) polymeric binder as a majorcomponent of the primer layer, (B) 0.1-1.5 wt. % of inverselysynthesized aliphatic polyurethane particle as iridescent reducingcomponent, and (C) at least one crosslinking agent, wherein, the haze ofthe polyester film is 4% or less.

2. The optically clear biaxially oriented polyester film of contemplatedembodiment 1. wherein the polymeric binder is polyester.

3. The optically clear biaxially oriented polyester film of contemplatedembodiment 1. wherein thickness of the primer layer is 0.03-0.15micrometer.

4. The optically clear biaxially oriented polyester film of contemplatedembodiment 1. wherein the crosslinking agent is selected fromcarbodiimide, melamine, aziridine, glyoxal, oxazoline or mixturethereof.

5. The optically clear biaxially oriented polyester film of contemplatedembodiment 1. wherein the crosslinking agent is carbodiimide.

6. The optically clear biaxially oriented polyester film of contemplatedembodiment 1. which further comprises at least one UV absorber.

7. The optically clear biaxially oriented polyester film of contemplatedembodiment 6. wherein the UV absorber is selected from2-hydroxybenzotriazoles, benzoxazinones, triazines and mixture thereof.

8. The optically clear biaxially oriented polyester film of contemplatedembodiment 6. wherein the UV absorber is2,4-bis-biphenyl-6-[2-hydroxy-4-(2-ethyl-hexyloxy)phenyl]-1,3,5-triazine.

9. The optically clear biaxially oriented polyester film of contemplatedembodiment 1. for a window film, optical, display, label or photovoltaic application.

10. This invention will be better understood with reference to thefollowing examples, which are intended to illustrate specificembodiments within the overall scope of the invention.

Test Methods

Haze:

Haze of films was measured according to ASTM D1003 that determines thepercent of transmitted light scattered at more than 2.5° from theincident light. A suitable instrument to measure haze is GARDNERHAZE-GARD PLUS No. 4725 hazemeter (BYK-Gardner USA). A haze value of 3%or less isconsidered acceptable, and 2% or less is preferred.

Iridescent Appearance Test:

Samples of primer layer-coated base layer composite films were coatedwith an acrylic polymer layer. A solution of acrylic polymer compositionwith a UV cure initiator in methyl ethyl ketone was drawn onto thecomposite film sample with a No. 2.5 mayer coating rod. The solutioncoated composite film was passed at a rate of 0.25 m/s (50 ft/min.)through a field of ultraviolet light radiation of 620,000 Watts/m² (400Watts per square inch) to cure and thereby solidfy the acrylic polymercomposition. The coated film was taped to a black & white lanetta card(9A). Then the surface of the hard coating was visually inspected underfluorescent lamp illumination and rated according to the scale below.

Rating 1 (Good): No iridescence observed.

Rating 2 (Acceptable): Weak iridescence observed.

Rating 3 (Unacceptable): Strong iridescence observed.

Acrylic polymer layer (hard coat) adhesion test (Spray Test):

A 12.7 cm×25.4 cm sample of composite film coated with an acrylicpolymer layer as described in the Anti-Iridescent Appearance testmethod, above, was rolled up in a cylinder of approximately 2.5 cmdiameter and secured with a paper clip. The rolled up film was exposedto temperature of 66° C. (150° F.) for 5 minutes. Thereafter thecylinder was unrolled and the surface of the film was visually inspectedand rated according to the scale below.

Rating 1 (Good): No discoloration and no separation of acrylic polymerlayer from composite film observed

Rating 2 (Acceptable): Discoloration but no separation of acrylicpolymer layer from composite film observed

Rating 3 (Unacceptable): Discoloration and separation of acrylic polymerlayer from composite film observed

Acrylic polymer layer (hard coat) adhesion test (Boil Test):

A 12.7 cm×25.4 cm sample of composite film was coated with an acrylicpolymer layer as described in the Anti-Iridescent Appearance testmethod, above. The acrylic polymer layer-coated sample was submerged inboiling water (100° C.) for 5 minutes. Thereafter the surface of thefilm was visually inspected and rated according to the same scale as inthe Spray Test for Acrylic polymer layer adhesion.

Acrylic polymer layer (hard coat) adhesion test (Tape Test):

A 12.7cm×25.4 cm sample of composite film was coated with an acrylicpolymer layer as described in the Anti-Iridescent Appearance testmethod, above. The acrylic polymer layer surface of the sample waspressed against Scotch Brand 810 adhesive tape to adhere the sample tothe tape. The tape was peeled away rapidly from the tape in directionsperpendicular and parallel to the tape . The sample and tape werevisually inspected for transfer of hard coat acrylic polymer layer tothe tape. The sample failed the test if any of the acrylic polymer layertransferred to the tape.

EXAMPLES

This invention is now illustrated by examples of certain representativeembodiments thereof, wherein all parts, proportions and percentages areby weight unless otherwise indicated. All units of weight and measurenot originally obtained in SI units have been converted to SI units.

Comparative Example 1

Making biaxially oriented polyester film with a primer layer (in linecoating method):

Four masterbatch compositions, MB-A through MB-D, of addtives wereprepared by individually blending additive components with polyethyleneterephthalate in the proportions shown in Table 1. MB-A, MB-B and MB-Cwere produced by adding the additive components to the reaction massduring polymerization of the PET. After synthesis, the PET with additivemasterbatch compositions were pelletized. MB-D was made by charging theadditive with PET pellets (0.65 inherent viscosity) to a twin screwextruder in which the masterbatch composition was melt blended and thenpelletized.

TABLE 1 Average additive Additive particle size Concentration Additive(μm) (wt %) MB-A CaCO₃ 1.1 1.0 MB-B AlO₂ 0.1 1.5 MB-C SiO₂ 2.0 1.0 MB-DUVA(*) Not applicable(*) 20 (*)ultraviolet light absorber2,4-bis-biphenyl-6-[2-hydroxy-4-(2-ethyl-hexyloxy)phenyl]-1,3,5-triazine

An outer layer composition for a polyester base layer film was made bymixing the masterbatch pellets with PET pellets of inherent viscosity0.6 in the proportions shown in Table 2. The combination of pellets wasdried to less than 100 ppm moisture content then melt-blended in anextruder.

TABLE 2 Component (wt %) PET 65 MB-A 22 MB-B 7 MB-C 2 MB-D 4

A core layer was formed by mixing 4 wt. % of MB-D pellets and 96 wt. %PET (inherent viscosity 0.65) pellets, drying the mixture to less than100 ppm moisture content, then melt-blending the dried pellet mixture inan extruder. The outer layer and core layer melt streams were thencontinuously co-extruded at a temperature of 285° C. through arectangular joining zone to form an A/B/Amulti-layered melt structure.The multi-layered melt curtain was quenched on a casting drum at 20° C.to form a base layer film. The film was oriented in the machinedirection by stretching to 3.3 times original length at 95° C. with aroller stretcher.

Formulation of the Primer Layer Composition:

A liquid primer composition was formed by combining and blending touniform composition 19.3 parts weight per hundred (“pph”) of a polyesterbinder dispersion, 0.04 pph of a leveling surfactant, 0.08 pph of anantifoam/leveling surfactant, 0.14 pph of an aqueous silica particledispersion, and 2.62 pph of an aqueous crosslinking agent dispersionwith 76.7 pph deionized water. The polyester binder dispersion was a 30wt % solids polyester particle dispersion in 2% propanol, aqueoussolution (Eastek® 1200, Eastman Chemicals Company, Kingsport, Tenn.).The leveling surfactant was an ethoxylated acetylenic diol (Surfonyl440), and the antifoam/leveling surfactant was an ethoxylated acetylenicdiol (Surfonyl 420). The silica particle dispersion was a 20 wt %aqueous dispersion of synthetic amorphous silica particles (Grace 703A,W. R. Grace Co.). The crosslinking agent was a 45 wt % aqueousdispersion of polycarbodiimide (Solucote XL1, (DSM NeoResins B.V.,Waalwijk, Netherlands).

The liquid primer composition was coated with a Mayer rod coater ontoone side of the base layer film. The primer solution was deposited at arate of about 1.4 g/m² of base layer area that was calculated aseffective to produce a primer layer of basis weight of 0.10 g/m², 0.1 μmthickness and 97.9 wt. % crosslinked polyester after drying. The wetcoated film was transported through an oven, preheated at 110° C., andoriented in the transverse direction to 4.0 times original width at 110°C. The composite film was heat-set at 236° C. and relaxed (5%) using achain driven stretcher. The completed film was then wound up. Thebiaxially oriented polyester film had an A/B/A thickness of 1.5/47/1.5μm. The film was aged seven days at ambient temperature for fullcrosslinking to occur.

Application of an acrylic polymer layer on the primed polyestercomposite film, above:

A methyl ethyl ketone (MEK) based, acrylic hard coat containing a UVcure inflator was drawn onto the primed surface of the substratepolyester composite film above with a size No. 2.5 Mayer coating rod.The solution coated composite film was passed at a rate of 0.25 m/s (50ft/min.) through a field of ultraviolet light radiation of 620,000Watts/m² (400 Watts per square inch). The radiation exposure caused thecrosslinking agent to crosslink the polyester binder while volatizingthe MEK from the solution to produce an acrylic polymer coated polyesterfilm (having a dry hard coat of acrylic polymer laminated onto the basepolyester film above).

Samples of the above coated film were subjected to adhesion, haze andanti-iridescence appearance testing. Analytical results are presented inTable 3, below. The product did not meet low iridescence specification.

Example 2

The procedure of Comparative Example 1 was repeated except that theliquid mixture composition for the primer layer consisted of 19.267 pphpolyester primer dispersion, 0.04 pph of leveling surfactant, 0.081 pphof antifoam/leveling surfactant, 0.144 pph of aqueous silica particledispersion, 2.615 pph of aqueous crosslinking agent dispersion and 0.329pph aqueous polyurethane particle dispersion combined with 76.455 pphdeionized water. The aqueous polyurethane dispersion was a 32 wt %solids dispersion of particles less than about 100 nm size of inverselysynthesized aliphatic polyurethane (Neorez R1010, DSM NeoResins B.V.,Waalwijk, Netherlands). The primer layer drying conditions were same asin Comparative Example 1. The dry primer layer basis weight was againabout 0.10 g/m² and dry primer layer thickness was about 0.1 pm.Composition of the dry primer layer was about 96.5 wt % crosslinkedpolyester and about 1.5 wt % of the inversely synthesized aliphaticpolyurethane particles dispersed therein.

Samples of the above acrylic polymer coated polyester film weresubjected to adhesion, haze value and anti-iridescence appearancetesting. Analytical results shown in Table 3 indicate that theiridescent rating value of 1 to 2 was acceptable to good and adhesionand haze specifications were satisfied..

Comparative Examples 3 and 4

In Comp. Ex. 3, the procedure of Example 2 was repeated except that thethe content of the polyurethane particle in the primer layer was 9.0 wt%. In Comp. Ex. 4, the procedure of Example 2 was again repeated exceptthat the the content of the polyurethane particle in the primer layerwas 0.22wt %.

Evaluation Result

Analytical results are shown in Table 3. These examples show thatexcessive amounts of polyurethane particles can detract from opticalclarity of the film (Comp. Ex. 3) and that insufficient polyurethaneparticles do not provide adequate iridescence canceling power (Comp. Ex.4).

Comparative Example 5

The procedure of Example 2 was repeated except that primer solution wasdeposited onto the base layer at about 2.8 g/m² rate such that theprimer thickness after drying was 0.2 μm. Analytical results shown inTable 3 indicate that excessive thickness of the primer layer adverselyaffects optical clarity although iridescence was canceled.

Comparative Examples 6 and 7

For Comp. Ex. 6, the procedure of Example 2 was repeated except that aglyoxal crosslinking agent (Freechem® 40DL from Emerald PerformanceMaterials) was substituted for the carbodiimide crosslinking agent.Freechem 40DL is an aqueous solution of ethanedial which containing lessthan 0.2% residual acid. For Comp. Ex. 7 the same procedure as Ex. 2 wasrepeated except that melamine formaldehyde resin crosslinking agent(Cymel® 385 Cytec Industries, Woodland Park, NJ) replaced thecarbodiimide crosslinking agent. The amounts of crosslinking agent wassame as Example 2. Analytical results are shown in Table 3. They revealthat the inversely synthesized aliphatic polyurethane and primerproportions as in Ex. 2 gave satisfactory iridescence and hazeproperties. However, replacement of carbodiimide by other crosslinkingagents gave inferior Spray and Boil adhesion performance.

TABLE 3 Comp. Comp. Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex.5 Ex. 6 Ex. 7 Spray 1 1 1 1 1 2 3 Ad- hesion Boil 1 1 1 1 1 2 3 Ad-hesion Tape Pass Pass Pass Pass Pass Pass Pass Ad- hesion parallel TapePass Pass Pass Pass Pass Pass Pass Ad- hesion perpen- dicular Irides- 31-2 1-2 3 1-2 1-2 1-2 cence Haze % 1.37 1.7 3.53 0.85 6.5 1.22 1.35

Although specific forms of the invention have been selected in thepreceding disclosure for illustration in specific terms for the purposeof describing these forms of the invention fully and amply for one ofaverage skill in the pertinent art, it should be understood that varioussubstitutions and modifications which bring about substantiallyequivalent or superior results and/or performance are deemed to bewithin the scope and spirit of the following claims. The entiredisclosure of the US patents and patent applications referred in thisapplication are hereby incorporated herein by reference.

1. A composite film comprising a core layer of substantiallyparticle-free and optically clear polyester, (a) a first outer layer indirect contact with one side of the core layer, (b) an optically clearprimer layer in direct contact with a side of the first outer layeropposite the core layer, and (c) optionally, a second outer layer indirect contact with a side of the core layer opposite the first outerlayer, in which each outer layer is optically clear, is independentlyabout 0.5-7 μm thick and independently comprises inorganic particlesdispersed uniformly in a polyester matrix, wherein the improvementcomprises the primer layer comprising inversely synthesized aliphaticpolyurethane particles uniformly dispersed in a matrix of polyestercrosslinked by a carbodiimide crosslinking agent.
 2. The composite filmof claim 1 in which both of the first outer layer and the second outerlayer are present.
 3. The composite of claim 1 which further comprisesan optically clear protective layer in direct contact with a side of theprimer layer opposite the first outer layer.
 4. The composite of claim 3in which the protective layer comprises greater than 50 wt. % of anacrylic polymer.
 5. The composite of claim 4 in which the protectivelayer is a devolatized residue of a solution of the acrylic polymerdissolved in an organic solvent from which solution the organic solventhas been substantially removed.
 6. The composite of claim 5 in which theprotective layer comprises a trace amount of the organic solvent.
 7. Thecomposite of claim 6 in which the organic solvent is methyl ethylketone.
 8. The composite of claim 5 in which the polyurethane particlesare present in the primer layer at a concentration such that thecomposite exhibits haze of at most about 3%.
 9. The composite of claim 8in which thickness of the primer layer is about 0.03-0.15 μm.
 10. Amethod of making an optically clear non-iridescent composite filmcomprising the steps of (A) providing a dispersion comprising inverselysynthesized aliphatic polyurethane particles of size in the range ofabout 1-100 nm and polyester binder particles dispersed of a polyesterresin having a softening point lower than 100° C. in a predominantlyaqueous medium comprising carbodiimide crosslinking agent dissolved inwater, (B) providing a base layer comprising a core layer of polyesterfilm of thickness in the range of about 1-500 μm and comprising about0.1 to 2 wt % of a UV light absorber composition, (C) depositing a wetcoating of the dispersion onto one side of the base layer, (D) heatingthe base layer and wet coating effectively to (i) soften the polyesterbinder, (ii) evaporate substantially all volatile components of thepredominantly aqueous medium, and (iii), and activate the crosslinkingagent, thereby creating an optically clear film of solidified primerlayer in contact with the one side of the base layer, in which the wetcoating on the the base layer is present in an amount effective toproduce a thickness of the solidified primer layer in the range of about0.03-0.15 μm.
 11. The method of claim 10 in which the inverselysynthesised aliphatic polyurethane particles are present in the primerlayer at a concentration effective to make the optically clearnon-iridescent composite film have a haze value of at most about 4%. 12.The method of claim 10 in which the step of providing the base layerfurther comprises the step of providing a first outer layer about 0.5-7μm thick and comprising inorganic particles uniformly dispersed in apolyester matrix, in which the first outer layer is in direct contactwith a side of the core layer opposite the one side of the base layerbearing the primer layer, such that the core layer and the first outerlayer collectively defining the base layer.
 13. The method of claim 12in which the step of providing the base layer further comprises the stepof providing a second outer layer about 0.5-7 μm thick and optionallycomprising inorganic particles uniformly dispersed in a polyestermatrix, in which the second outer layer is in direct contact with a sideof the core layer opposite the first outer layer, such that the corelayer, first outer layer and second outer layer collectively define thebase layer.
 14. The method of claim 12 which further comprises the stepsof (E) providing a solution comprising acrylic polymer, a cureinitiator, and an amount of organic solvent effective to dissolve theacrylic polymer, (F) placing a uniformly thick solvent-wet coating ofthe solution onto a side of the primer layer opposite the base layer,(G) heating solvent-wet coating effectively to evaporate substantiallyall volatile components of the organic solvent, thereby creating anoptically clear film having a protective layer of solidified acrylicpolymer in contact with the primer layer, in which acrylic polymercomprises greater than 50 wt. % of the primer layer.
 15. The method ofclaim 14 in which the protective layer is effective to make theoptically clear non-iridescent composite film have a haze value of atmost about 3%.